The present invention relates to electrical cartridge heaters and in particular to a cartridge heater having a protective low friction coating.
Cartridge heaters are used to transmit heat to a wide range of equipment and apparatuses. Cartridge heaters are generally cylindrical and adapted to be inserted in appropriately dimensioned bore holes in apparatuses to be heated. A typical cartridge heater comprises a helically wound electrical heating element connected to two electrical leads and enclosed in a cylindrical or square metal sheath or casing, commonly made out of stainless steel.
In use, it is desirable to obtain a close fit between the cartridge heater and the bore hole in which the heater is inserted to ensure maximum heat transfer from the cartridge heater to the apparatus. A common problem in using currently available cartridge heaters in tight fitting bore holes is that the heaters often seize in the bore holes after operation. It is believed that seizure of the cartridge heater in the bore hole is caused by oxidation between the stainless steel casing of the cartridge heater and the metal of the bore hole. Seizure of the cartridge heater in the bore hole may also be caused by warping of the cartridge heater in the bore hole. A seized cartridge must be driven out of the hole or in some cases drilled out. The heater is usually destroyed in this process and the equipment in which the cartridge heater was seized may be damaged in removing the cartridge.
Attempts have been made to facilitate removal of cartridge heaters from the bore holes in which they have been used. Liquid lubricants, such as oils, greases and solutions of magnesium and water have been applied to the stainless steel casings of cartridge heaters prior to insertion into a bore hole to facilitate later removal. Although liquid lubricants are commonly used they prove to be inadequate.
If liquid lubricants are to be used, the user must purchase the cartridge heaters and the lubricants separately. The user must apply the lubricant themselves which can be a messy operation and if not properly supervised can be done inadequately. Due to the close fit between the cartridge heater and the bore hole, insertion of the cartridge heater in the bore hole often results in the liquid lubricant being forced away from the surface of the cartridge heater, exposing the surface of the cartridge heater to the metal defining the bore hole such that the metal to metal contact would be subject to oxidation. Also, once the lubricated cartridge heater is inserted in the bore hole, the liquid lubricant would tend to migrate toward low spots exposing the metal surface of portions of the cartridge heater to direct contact with the metal defining the bore hole so as to allow oxidation.